1. Field of the Invention
The present invention relates to a data correction apparatus, a data correction method, a magnetic resonance imaging apparatus and an X-ray CT (computed tomography) apparatus which correct data showing a spatially nonuniform SNR (signal to noise ratio) distribution due to a sensor's sensitivity showing a spatial distribution so that the corrected data are uniform.
2. Description of the Related Art
A MRI (Magnetic Resonance Imaging) apparatus is conventionally used as a monitoring apparatus in a medical field (see, for example, Japanese Patent No. 3,135,592).
The MRI apparatus is an apparatus which generates gradient magnetic fields by gradient coils on an imaging area of an object set in a cylindrical static field magnet for producing a static magnetic field, resonates nuclear spins in the object magnetically by transmitting RF (Radio Frequency) signals from an RF coil and reconstructs an image of the object by using NMR (Nuclear Magnetic Resonance) signals generated due to an excitation.
In the MRI apparatus of recent years, for speeding up of the imaging, an RF coil is structured of a whole body (WB) coil for transmission and a phased-array coil for reception. The phased-array coil includes a plurality of surface coils, so that it is possible to reduce the imaging time as the respective surface coils receive the NMR signals at the same time to obtain more data in a short period of time.
However, when the RF coil is structured of the phased-array coil and the WB coil, signal intensities of the image data obtained through a reconstruction processing together with the NMR signals have also nonuniformity depending on the nonuniformity of the sensitivity of the phased-array coil or the WB coil. In general, the nonuniformity of the sensitivity of the WB coil is sufficiently small at an ignorable level. However, in particular, the nonuniformity of the sensitivity of each surface coil in the phased-array coil used for each purpose is large and affects the image data.
For this reason, it is necessary to correct the nonuniformity of signal intensities of the image data due to the nonuniformity of the sensitivity of the phased-array coil.
In view of the above, up to now, prior to the main scan for generating the image of an object, the sensitivity pre-scan is executed. Then, through the sensitivity pre-scan, the image data is acquired from each of the phased-array coil and the WB coil. On the basis of the signal intensity ratio that is a division value of the signal intensities of pieces of image data, the sensitivity distribution of the phased-array coil is estimated as the three-dimensional sensitivity map data. Furthermore, the signal intensity unevenness of the image data is corrected with use of the thus acquired three-dimensional sensitivity map data of the phased-array coil.
However, in the MR imaging while using the above-mentioned plural surface coils and the MR imaging while using the single surface coil, when the sensitivity distribution of the surface coil is corrected, there is a problem in that a spatial nonuniformity occurs in the SNR. That is, before the correction, the sensitivity distribution of the surface coil is spatially nonuniform but the image noise level is constant.
Therefore, if the sensitivity distribution of the surface coil is corrected and the signal intensity of the image data depending on the space is set constant, the image noise becomes spatially nonuniform. For example, the image noise at a part where the signal intensity is amplified through the correction of the sensitivity distribution has larger intensity than the image noise at a part where the signal intensity is not emphasized. As a result, the SNR becomes spatially nonuniform, which leads to the degradation in the image quality, and thus the spatial nonuniformity of the SNR is not preferable in the diagnosis.
In addition, in a medical device such as other image diagnostic apparatus or a biological information measuring instrument other than the MRI apparatus using the surface coils as the sensors as well, if the intensities of the signals obtained while the spatially nonuniform sensitivity distribution of the sensor is collected are set constant, the spatial nonuniformity occurs in the SNR together with the noise, which may lead to the degradation in the quality of image or the measurement result.